This is a presentation of the N. W. Harris lectures for 1912, which were given by Prof. M. J. Rosenau, under the auspices of the Northwestern University. Why we have a milk question is fully explained in the first chapter. The author takes a sound attitude toward the exaggeration of the danger from milk, and his motto is "to enlighten, not to frighten." The presence of harmless as well as harmful bacteria is emphasized, since large numbers of harmless bacteria gain access to milk, so that nambers alone do not constitute danger. Special attention is given to pasteurization of milk, and although the author states that clean natural milk can never be replaced by treated milk, he admits that pasteurization has been forced on us and that clean, safe milk is at present problematic. In this connection he discusses the advantages and shortcomings of medical milk commissions and points out some disadvantages